This invention relates to the field of composite structural materials, and particularly to metal matrix composite materials.
Performance requirement goals for future advanced airframe structures and gas turbine engines exceed the capabilities and limits of currently available materials and manufacturing technologies. Improvements in lightweight, high-temperature materials and processes are required to meet the challenging goals. Metal aluminides, particularly titanium aluminide base alloys, offer opportunities for weight reduction compared to nickel base superalloys. To achieve the ambitious high temperature capability goal in a light and stiff material, it has been proposed to fabricate fiber-reinforced composites using titanium aluminide base alloys as the matrix. However, as high strength and high temperature matrix materials are selected to provide high performance composites, it becomes more difficult to fabricate the composites because the temperatures and pressures required to consolidate the materials also increase.
Composites can be fabricated by placing a reinforcing material such as silicon carbide fibers between foils of a matrix material such as a metal alloy. These ingredients are then consolidated into a composite by pressing them together at a temperature and pressure which will cause the matrix to flow around the reinforcing fibers and diffusion bond the matrix together.
An alpha titanium aluminide (Ti.sub.3 Al) base alloy is currently available (Ti-24Al-11Nb, atomic %). Alloys using other titanium aluminides (gamma-TiAl and near delta-TiAl.sub.3) and using other metals to form the aluminide such as nickel aluminide and iron aluminide are also under development. Many reinforcing phases are also available in the form of fibers, powders, and whiskers made from silicon carbide, alumina, graphite, boron and other materials. Some of these reinforcing phases have surfaces which are modified to promote their incorporation into metal matrix composites. For example, a silicon carbide fiber was modified with the goal of withstanding the thermal exposure required to consolidate and form titanium matrix composites ("A Review of SiC Filament Composite Production and Fabrication Technology", J. A. Cornie, Fourth Metal Matrix Composites Technology Conference, Proceedings, MMCIAC-Kaman Tempo, Santa Barbara, Calif., pgs. 30-1 though 30-9, 1982). It has however been found that this C-rich outer layer (SCS-6) does not prevent chemical reaction with the matrix, but protects the CVD SiC fiber from notching and damage.
In order to obtain a sound composite with optimum mechanical properties, it is necessary to consolidate the matrix with the reinforcement phase without leaving cracks and voids in the composite, and without damaging the reinforcement by mechanical stress and by formation of brittle phases due to chemical reaction with the matrix at the consolidation temperature. This is a particular problem when high strength matrices such as titanium aluminide alloys are used with reinforcing materials which are brittle and which tend to react chemically with the matrix material.
FIG. 1 is a photomicrograph of a prior art composite showing voids 2 between the reinforcing phase 4. During consolidation, the matrix material 6 was unable to flow between the closely spaced reinforcing fibers, and consequently voids were left. Such voids can reduce the integrity of structures made from the composite. Attempts to fill such voids by increasing the temperature and pressure of consolidation can cause other problems such as fiber breaking or chemical reaction of the reinforcing fibers with the matrix.